Flooring a car from off idle would be less consistent than leaving on the converter from an ET stand point, because there is a longer dynamic ramp to full throttle that is occuring, vs. a static motor held against a converter at a higher throttle opening, Less variables go into the equation.
The only reason not to leave with a loaded converter is it can blow the tire s off, so one would leave at a lower RPM than fully stalled.
Switching to a trans brake it will be leaving at a similar loaded torque converter state as foot brake so blowing the tires off is now a concern, that wasn't a concern leaving off idle.
I would make sure the car can take a fully loaded torque converter foot brake launch (from a traction standpoint) before switching to a trans brake.